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问题 问答题

一个平抛物体，在落地前的最后1s时间内，速度方向由与水平方向成45°角变为与水平方向成53°角．g取lO m/s2，求：

（1）抛出时的初速度V0；

（2）抛出时离地面的高度．

答案

（1）由于平抛运动在水平方向的速度的大小是不变的，

设水平初速度为V0则，速度方向与水平方向成45°角时，竖直速度的大小为V0，

当速度方向与水平方向成53°角时，竖直速度的大小为

4
3
V0 ，

在竖直方向做自由落体运动，由速度公式可得，

4
3
V0=V0+gt，

其中时间t=1s，

所以 V0=30m/s，

即抛出时的初速度V0为30m/s．

（2）落地时竖直方向上的速度大小为

4
3
V0=40m/s，

由V2=2gh 可得，

抛出时离地面的高度h=

V2
2g
=
402
20
=80m．

单项选择题

对肺栓塞的诊断的灵敏度上，肺灌注显像较X线平片为（）

A.明显高

B.明显低

C.相仿

D.两者均不灵敏

E.均灵敏
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单项选择题

Ravi, like many project (126) , had studied the waterfall model of software development as the primary software life-cycle (127) . He was all set to use it for an upcoming project, his first assignment. However, Ravi found that the waterfall model could not be used because the customer wanted the software delivered in stages, something that implied that the system had to be delivered and built in (128) and not as (129) .
The situation in many other projects is not very different. The real world rarely presents a problem in which a standard process, or the process used in a previous project, is the best choice. To be the most suitable, an existing process must be (130) to the new problem. A development process, even after tailoring, generally cannot handle change requests. To accommodate change requests without losing control of the project, you must supplement the development process with a requirement change management process.

A．parts

B．modules

C．software

D．a whole
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